Aslide9 Receivers 1

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Chelmsford Amateur Radio Society

Advanced Licence Course

Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004

(4) Receivers-1 - Parameters


Advanced Course(4) Receivers

Part-1 Receiver Parameters


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Receiver Parameters

Important performance measures for receivers

Frequency stability

Selectivity

Bandwidth

Sensitivity

Dynamic range

Signal-to-noise ratio (SNR)

Effect of RF amplifiers & pre-amps

Transverters


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Frequency Stability

Frequency stability is the same as for transmitters

Accuracy of tuning to an entered or displayed frequency

Ability to remain on frequency without drifting off

Often given in ppm parts-per-million

1ppm error at 28MHz is 28Hz.


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Selectivity

Selectivity is the ability to separate the wanted signal from nearby

unwanted signals (other stations)

Frequency MHz

Amplitude

Wanted signal

(weak)

Unwanted signal

(strong) Filter response


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Selectivity Measures

Measures of selectivity

60-dB bandwidth

adjacent channel rejection ratio (VHF, UHF channelised)

but how far away is the next channel (12.5kHz?)

For SSB, may specify opposite sideband rejection

Frequency MHz

Amplitu

de

0dB -Filter response

-60dB - 60dB bandwidth


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Bandwidth

Band of frequencies which the receiver should accept

Eg. CW (A1A) morse typically 300Hz

SSB uses 2.5 to 3kHz

VHF FM typically 7.5 or 15kHz

Usually 3dB BW specified, but not always!

Frequency MHz

Amplitude

0dB -Filter response

-3dB - 3dB bandwidth


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Sensitivity

Sensitivity defines the limit of detection of weak signals.

Receivers must have enough gain to bring weakest signals to

comfortable level.

the gain does not define sensitivity

Sensitivity is determined by 2 factors:

Bandwidth of the receiver

the wider the bandwidth, the more noise power it lets in

Noise figure of the receiver front-end

a noisy receiver needs more signal to overcome the noise

Receivers bandwidth should match the transmitted bandwidth.

so as not to exclude any signal or accept unnecessary noise.


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Sensitivity Definitions

Sensitivity is defined as the receiver input signal level for a given

SINAD at the output

eg. 0.2V for 12dB SINAD

SNR is Signal-to-noise ratio

SINAD is Signal + Noise + Distortion

Intelligible speech needs about 12dB SINAD

PowerNoise

PowerSignalSNR

DistortionNoiseDistortionNoiseSignalSINAD

Usually expressed in dB


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Dynamic Range

Dynamic range is the range of signal levels between the smallest

and greatest a receiver can handle

Lower limit set by sensitivity

Upper limit set by distortion or AGC control range

In practice, we are more concerned about dynamic range to

handle unwanted out-of-band signals (AGC doesnt apply).

How large an unwanted signal will it reject without affecting

sensitivity to wanted signals

Overload level may be specified for receiver front-end (RF

amplifier, mixer) as the 1dB compression point


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Dynamic Range

1dB compression point

Power level where amplifier gain drops by 1dB

Input level

Outputlevel

1dB

Amplifier linearity curve

Input compression

point

Output compression

point

Noise floor


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Noise Figures

There is a limit of physics to receiver sensitivity

Even for perfect receivers that add no noise (0dB NF)

Real receivers can get within a few dB of the limit

Typical noise figures

HF receiver; 12 to 20dB - not as critical as atmospherics dominate

VHF receiver; 6 dB

Microwave receiver; 2dB

Raw sensitivity is traded for dynamic range and selectivity in

environments where these are more important

Adjusting RF gain can optimise sensitivity vs. dynamic range

Expressed in dB

outputatSNRinputatSNRNF


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RF Amplifiers

If the RF pre-amp has a similar noise figure to the receiver

Sensitivity not improved, dynamic range made worse

If the RF pre-amp has a better noise figure to the receiver

Sensitivity improved, dynamic range still worse

If there is a feeder loss before the receiver (masthead amp)

Without preamp, receiver performance degraded by loss

Amplifier can overcome feeder loss, performance improved

RF pre-amplifier Receiver


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Downconverters

Downconverters convert signals from one band to another.

This enables reception of signals out of the tuning range of the

receiver.

Use of downconverters common for VLF, microwave, and weathersatellite.

Operator must mentally add the frequency offset to the frequency

displayed by the receiver.

Down-

converterReceiver


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Transverters

Transverters are bi-directional frequency converters

May be used for multimode (SSB J3E , CW A1A) operation by

using a HF transceiver on VHF or UHF

Displayed frequency must be mentally adjusted

Reliable high isolation switching is vital

Down-

converter

Up-

converterTrans-

ceiver

Power

amp

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